In conventional wire-cut electrical discharge processors in which a part or product is cut off by spark discharges occurring between a wire electrode and a workpiece of cemented carbide, hardened steel and so on, a wire electrode before the onset of electrical discharge is threaded through a starting hole pierced in advance in the cut-out part or workpiece and, while wire electrical discharge machining is being done, is fed constantly into a discharge cutting zone where the part or product is made in any desired shape. The workpiece in, for example immersion wire electrical discharge machining process, is immersed in the medium and clamped on a worktable. With the electrical discharge machining process of the type in which the wire electrode is threaded through the discharge cutting zone in the part or workpiece, the part or product is cut off from the workpiece in a phase the electrical discharge machining circuit is held closed.
A conventional wire-cut electrical discharge machining process is disclosed in, for example JP patent No. 3 366 509 in which the first machining phase and the second cut-off phase are both carried out with only one processing program. With the prior wire-cut electrical discharge machining process, a pathway program, uncut amount and reversing amount to cut off every male pattern are set and input. The first machining phase comes to rest with leaving the uncut amount and stores the spot. In the second machining phase to cut off the uncut amount, the wire electrode after having gotten back from the stored spot along the cutting path or kerf by the programmed uncut amount makes automatic connection and resumes the electrical-discharge at the site to cut off the male pattern.
In Japanese Laid-Open Patent Application No. 2000-280 124 which is a commonly-assigned senior application, there is disclosed a wire electrical discharge machining process to cut out a part or product on the basis of a machining program set previously. The uncut distance left shortly before completion of machining is determined in relation to the shape of specific part or product according to the previously set machining program. With the prior wire electrical discharge machining process, the programming for machining process can be swift and simple. Moreover, the determination of the uncut distance best for the machined shape is proved effective in certainly preventing the product from falling apart from the workpiece.
In Japanese Patent No. 3 521 283 which is a commonly-assigned senior application, there is disclosed a wire electrical discharge machining equipment in which the small part or product is automatically ejected following the machining phase. The control unit for wire-electrode travel, while moving the wire electrode based on the predetermined machining pathway, controls the electrical discharge spacing between the wire electrode and the workpiece, outputting an alarm signaling the completion of retreating phase to cease the machining operation, making the alarm signal invalid after the wire-electrode travel has ended, and activates the ejector controller after the end of the wire-electrode travel to eject the cut-out part out of the workpiece.
In another Japanese Patent No. 4 480 822 which is a commonly-assigned senior application, the immersion wire electrical discharge machining processor is disclosed in which the buoyancy member supports the part or product cut out from the workpiece to prevent the short-circuit which would be otherwise occur between the cut-out part and the wire electrode, thereby supporting a heavy part in safety. The buoyancy member less in specific gravity than processing liquid is underneath the workpiece. The buoyancy developed in the buoyancy member owing to the processing liquid floats the parts cut out from the workpiece by the wire electrode so as not to sink in the processing liquid.
With the wire electrical discharge machining process as stated earlier, however, the workpiece isn't cut across a matter of more than a few millimeters in the first machining phase. Subsequently, the machining process shifts to the next cutting phase to shape a desired contour of the part and then isolate completely the part from the workpiece. The part is further transferred to finish-cut phase. With machining process as stated just above, the part with uncut spots has to be cut out from the workpiece by the electrical discharge process. Upon the prior electrical discharge machining to produce the part of preselected shape, the part has to be cut out with leaving uncut spots to keep the part against falling apart away from the workpiece or scrap. Thus, the prior electrical discharge machining has need of an additional step to isolate finally the part apart from the workpiece and an additional device for recovery of the products of workpiece. When leaving intact the uncut spots in return for isolation of the part, there is a risk that the part going to be cut out from the workpiece gets tilted relative to the clamped workpiece shortly before the end of machining process, whereby the short circuit occurs between the wire electrode and the workpiece, causing an interruption of electrical discharge and/or an abnormal discharge, which would cause damages to the processed surface of the workpiece. When completely cutting out the part from the workpiece, there would be an adverse possibility that the workpiece couldn't be attracted magnetically or the workpiece, even though attractable magnetically, is too heavy in weight to be supported with any practical magnetic force.
Looking at the electrical discharge phenomenon of a qualitatively long interval which causes a breakdown between the metal electrodes lying in a gaseous insulator, it was found that the breakdown starts with a corona discharge phase, then going through a spark discharge phase and an arc discharge phase in sequence, and ends in the breakdown. Thus, it has been developed to control voltage-current characteristics so as to conduct the discharge machining at the spark discharge and the arc welding or plasma welding at the arc discharge or plasma discharge. Based on the consideration of the discharge phases as stated just above, the technical concept is motivated in which the wire electrode is used for the electrical discharge machining to cut out the part from the workpiece and for arc welding to make a coalescence of the part with the workpiece.